Conventionally, as an apparatus for inspecting the appearance of the surface of an inspection object as mentioned above, the apparatus disclosed in the Japanese Unexamined Patent Application Publication No. 2004-317126 is known, for example.
In this apparatus, a laser slit beam is irradiated on the surface of an inspection object, the irradiated laser slit beam is imaged by an appropriate imaging device, and information on the height of the surface of the inspection object is obtained by analyzing the obtained image according to a light-section method. On the basis of the obtained height information, flaws, chips and the like which are present in the surface of the inspection object are detected and further the volume of the inspection object is calculated.
By the way, in inspection of this type which uses a light-section method, there is a problem that, when a laser slit beam is imaged from only one direction, there may occur some blind spots which cannot be seen in the imaging direction depending on the surface shape of the inspection object and accurate inspection cannot be performed for these blind spots because reflected lights of the laser beam cannot be received. As shown in FIG. 18, if a defect 200 is present in the surface of an inspection object K, there occur a blind spot 200a when the inspection object K is imaged by a camera 201 from the direction indicated by the solid line. However, the blind spot 200a can be imaged by imaging the inspection object K from the opposite direction (the direction indicated by the two-dot chain line).
Therefore, the applicant of the present application has suggested, in the Japanese Patent Application Nos. 2009-281084 and 2009-281087, an appearance inspection apparatus in which the laser slit beam is imaged from the front and from the rear in the direction of conveying the inspection object.
This appearance inspection apparatus, as shown in FIG. 19, has, as a image capturing device 110, an area sensor camera 111 disposed above a conveyance path of a straight conveying section 100, a slit beam emitter 112 emitting a band-shaped slit beam, mirrors 113, 114 for guiding the slit beam emitted from the slit beam emitter 112 toward just below the area sensor camera 111 to irradiate the slit beam on an inspection object K being conveyed by the straight conveying section 100, mirrors 115, 116 for receiving a reflected light of the slit beam irradiated on the inspection object K on the downstream side in the conveyance direction of the straight conveying section 100 (the direction of the arrow) and guiding it into the area sensor camera 111, and mirrors 117, 118 for receiving a reflected light of the slit beam on the upstream side in the conveyance direction and guiding it into the area sensor camera 111.
The area sensor camera 111 has an area sensor comprising picture elements arranged in lines and columns, and, as shown in FIG. 20, images of the two reflected lights are formed on the area sensor in a state of being aligned in the direction orthogonal to the raster direction of the area sensor (in a state of being aligned vertically) within the region of the area sensor (the region indicated by the one-dot chain line). It is noted that, in such a conventional apparatus that a slit beam is imaged from the upstream side and from the downstream side in a conveyance direction using two mirrors on each side, images thereof are necessarily formed on the area sensor in the state of being aligned vertically.
For lines within a preset width, for example, line widths A and B shown in FIG. 20, the area sensor camera 111 scans in the raster direction and outputs, for each of the line widths A and B, image data comprising pixels arranged in lines and columns with luminance data, and the output image data are used for inspection.